Interest in molecules which consist of a donor part (D) linked via an extended .pi.-electron bridge to an acceptor part (A) stems from their potential nonlinear optical applications. They are capable of exhibiting second harmonic generation but for their second order molecular hyperpolarisabilities (.beta.) to be preserved as a bulk property it is necessary to suppress the tendency of the molecules to stack centrosymmetrically.
Most materials with large second-order nonlinearities form centrosymmetric (Y-type) Langmuir-Blodgett films, an example being E-N-docosyl-4- 2-(4-dimethylaminophenyl)ethenyl! pyridinium bromide, a hemicyanine dye reported by Gitling et al. (Thin Solid Films 132, 101, 1985). For second harmonic generation it is necessary for the films to be noncentrosymmetric (X-type or Z-type). We have discovered that E-N-octadecyl-4- 2-(4-dimethylaminophenyl)ethenyl! pyridinium iodide generates a more intense second harmonic signal than has been obtained previously from hemicyanine dyes. We believe that this compound is novel and it forms one aspect of our invention. Langmuir Blodgett films which comprise this novel hemicyanine dye are also believed to be novel and they constitute a second aspect of this invention.
However, only relatively modest second harmonic intensities have been obtained from monolayer films of the hemicyanine dyes including the dye referred to above and in each case, the signal diminishes for the bilayer.
To overcome this problem attempts have been made to interleave the optically active layers with spacers, generally a fatty acid. The technique has met with limited success, one of the best examples so far being N-ethyl-N-octadecylaminostilbazolium poly-(epichlorohydrin) interleaved with behenic acid (Anderson et al., Thin Solid Films 179, 413, 1989); the second harmonic intensity from 25 bilayers is 265 times that from the monolayer, the signal falling short of the theoretical quadratic dependence, I.sub.(n) =I.sub.(l) n.sup.2 where n is the number of active layers.